The present invention relates generally to devices and methods for casting of engine components, and more particularly to an advanced bi-metallic piston and a way to manufacture the same.
The body of a piston used in internal combustion engines (ICEs) is typically made up of a cylindrical-shaped head (also called a dome), from which a downward-extending skirt will have one or more ring grooves formed in it, as well as lands between the grooves that are of the same radial outward dimension as the outward-facing surface of the remainder of the skirt. Such components are typically made from lightweight materials, and from relatively low-cost forming techniques; in a preferred form, pistons are made from a cast aluminum-based alloy. While they are subjected to high combustion temperatures and pressures during engine operation, increasingly stringent emissions and efficiency requirements dictate that pistons of the future will need to be designed to withstand even more demanding operating conditions. This in turn will necessitate the use of higher-temperature capable materials and damage-resistant designs. This is especially true for pistons used in diesel engines that, in addition to being the predominant engine form for larger, commercial vehicles, are increasingly being used to power passenger vehicles. Likewise, high-revving gasoline engines (such as four cylinder engines, especially those with turbochargers), in an attempt to simultaneously maximize power output and minimize fuel consumption, have an ever-increasing reliance upon hotter-burning environments and faster reciprocating components, both of which place additional burdens on engine component and material designs.
While all of the various piston components mentioned above are expected to be subjected to additional thermo-mechanical loads as more power is extracted from smaller structures, it is the dome which, by virtue of being directly exposed to the combustion process, can be expected to be particularly vulnerable to damage. Advancements in the understanding of combustion dynamics taking place in a chamber created by the piston and the cylinder has led to more complex-shaped piston dome designs, where alternating regions of valleys and risers result in an undulated dome surface. These improvements in combustion dynamics further hamper the use of lightweight alloys and their limited mechanical and temperature capability where aluminum alloys, which have conventionally been used for weight reduction in diesel engine pistons, have limited thermal and mechanical durability that makes them incompatible with the higher temperature requirements of a more complete (and therefore higher temperature) combustion process. Steel pistons have the capability to endure the extreme environment; however, they are heavier than aluminum pistons. This weight problem is exacerbated by the high rate of speed and acceleration associated with piston movement, meaning that ancillary structures may additionally have to be fortified, with an even more detrimental weight impact.
Attempts have been made to combine the heat resistance of high temperature-capable materials with the lower weight of aluminum-based materials in diesel pistons. However, although composite pistons may satisfy the above objectives, the difficulties associated with their manufacture have offset many of their benefits. This is especially so because pistons have long been made as cast parts with some post-casting machining or related modifications. As such, it has been difficult to combine the inherent low-cost approach of casting with the flexibility of tailored material placement in the piston.
As with the diesel pistons discussed above, the dome of a gasoline engine piston is also exposed to extreme temperatures and, depending on the dome design (e.g. relatively flat or highly featured) may be subject to the same type of cracking as a diesel piston with a combustion bowl or other surface irregularities. Additionally, gasoline engine piston domes are typically thinner than diesel piston domes, and therefore more likely to transfer heat to other regions of the piston—such as the top ring groove—very quickly. Such increased exposure would increase the need for additional measures in the ring groove, such as anodizing the ring groove or using a high-temperature metal insert to provide adequate strength. Cooling approaches may also be employed. Oil galleries may be used in both diesel and gasoline pistons (especially turbocharged versions of the latter) to cool ring grooves and other portions of the piston that are exposed to high heat loads. While features such as ring groove inserts and oil galleries provide valuable cooling functions, their inclusion exacerbates the cost and complexity of such high-work pistons.